A conventional bi-directional DC-to-DC converter for exchanging electric power between direct-current power supplies having different voltages is proposed in, for example, Patent Literature 1.
FIG. 9 is a circuit diagram of conventional DC-to-DC converter 500 disclosed in Patent Literature 1. DC-to-DC converter 500 is a bi-directional DC-to-DC converter for exchanging electric power between direct-current power supplies having different voltages. A direct-current power supply such as an automobile battery is connected to low-voltage-side terminals 101 and 103. Another direct-current power supply such as an automobile generator is connected to high-voltage-side terminals 105 and 107. Transformer 109 is connected between low-voltage-side terminals 101 and 103 and high-voltage-side terminals 105 and 107.
Low-voltage-side switch 111 is inserted between transformer 109 and low-voltage-side terminals 101 and 103. High-voltage-side switch 113 is inserted between transformer 109 and high-voltage-side terminals 105 and 107. Each of low-voltage-side switch 111 and high-voltage-side switch 113 includes a bridge circuit made up of four switching elements such as field-effect transistors (hereinafter, FETs).
LC resonant circuit 115 is inserted between a high-voltage-side winding of transformer 109 and high-voltage-side terminals 105 and 107.
Output smoothing capacitor 117 is connected between low-voltage-side terminals 101 and 103, and output smoothing capacitor 119 is connected between high-voltage-side terminals 105 and 107.
Next, an operation by bi-directional DC-to-DC converter 500 to supply electric power from low-voltage-side terminals 101 and 103 to high-voltage-side terminals 105 and 107 is described. Note that an operation to supply electric power in the opposite direction is similar to the operation described below.
Out of the four FETs in low-voltage-side switch 111, a pair of FETs on the upper left and the lower right and a pair of FETs on the upper right and the lower left are alternately turned ON and OFF, with the result that a positive voltage and a negative voltage are alternately applied to a low-voltage-side winding of transformer 109, inducing positive and negative square-wave voltages in the high-voltage-side winding of transformer 109. Since high-voltage-side switch 113 functions as a rectifying device, the positive and negative square-wave voltages are rectified by high-voltage-side switch 113. Electric power is output from high-voltage-side terminals 105 and 107 via LC resonant circuit 115. Current outputted from transformer 109 becomes a sine wave current by LC resonant circuit 115. With this, it is possible to set the timing of turning OFF the FETs to around a zero cross point at which the current value is approximately zero, meaning that switching of the FETs is possible at around the zero cross point of the current value so that switching loss upon exchanging electric power can be reduced.